Learning WML & WMLScript 2000

Learning WML & WMLScript

The next generation of mobile communicators is here, and delivering content to them will mean programming in WML (Wireless Markup Language) and WMLScript, the languages of the Wireless Application Environment (WAE). The WAE allows information in almost all applications to be formatted for display on mobile devices, such as cell phones, and enables the user to interact with the information.

Why learn yet another technology? According to some estimates, 75 percent of web document viewing by the year 2002 will be through non-desktop devices, many using wireless technologies. Clearly, the future is wireless. For web developers who want to get up to speed quickly in these languages, Learning WML & WMLScript maps out in detail the WAE and its two major components, WML and WMLScript.

Fortunately, the WAE provides a World Wide Web-like model for writing applications, incorporating several key features of the Web to ease the transition for developers. Almost all wireless applications can be written with WML, which replaces HTML in the wireless environment, and WMLScript, which replaces JavaScript. With this book, web developers with some knowledge of programming and C, Java, or JavaScript syntax can easily master both languages.

Chapter by chapter, Learning WML & WMLScript takes readers through the following WML topics:
• Decks, templates and cards
• User Interaction
• Variables and contexts
• Tasks, events, and timers
• Text and text formatting
• Images

WMLScript topics include:

• Data types, conversions and variables
• Operators and expressions
• Statements
• Functions
• Standard libraries

Learning WML & WMLScript is the resource of choice for application developers who want to upgrade their skills and their sites so they won't be left plugged in during the wireless revolution.

Preface

The Wireless Application Protocol (WAP) is an industry-wide specification for applications that operate over wireless networks, with particular emphasis on applications for mobile devices, especially mobile phones. The standards are published by the WAP Forum, a body formed in June 1997 by Ericsson, Nokia, Motorola, and Unwired Planet, which has since been joined by over a hundred other companies, including IBM, Hewlett-Packard, Visa, and Microsoft. According to the WAP Forum's official statistics, WAP Forum members represent over 90% of cellular-phone manufacturers worldwide.

WAP will be supported by many kinds of devices, from simple cell phones similar to those available today to a new generation of "smart" phones with large touch screens and many applications of their own. That's before we even look at the range of personal digital assistants (PDAs) available, from palmtops to full miniaturized computers complete with keyboards. All will eventually get WAP capabilities, either directly from the manufacturer or as some sort of third-party upgrade. Each device has a different kind of display and different methods for user input. The job of the WAP specification is to sort out this mess and provide a common framework to allow applications to run across all these different platforms.

Since WAP works in a mobile environment, it also has to contend with the particular problems of wireless networks: low bandwidth (9600 bps or less is commonplace), high latency (round-trip times of the order of seconds are not uncommon), and unreliability (someone may be using her WAP phone when the train goes into a tunnel or when she walks past a tall building). Everyone with a mobile phone knows about the reliability problems.

These problems are why WAP is necessary. Some people may ask why they can't just have normal web pages delivered over normal TCP/IP connections, the only difference from their PCs at home being the wireless link. The trouble with the normal web technologies is that they are aimed at high bandwidth and big screens. WAP keeps the Internet model but optimizes each component for a mobile environment. It keeps track of the state of a session in case the connection is lost, provides compressed formats for the transferred data, and handles displaying of applications regardless of the input and output available.

The WAP Stack

The term that refers to all the different parts of the WAP specifications is the WAP stack. This is because the components can conceptually be thought of as layers in a stack. The user interacts with the top of the stack, and the communications hardware sits below the lowest level. This concept is illustrated in Figure P.1, together with the names of the various specifications involved.

Going from bottom to top, the lowest layer in the figure contains various protocols (the so-called bearer protocols), which aren't actually part of WAP but which provide the link between WAP and the actual communications hardware:

• IP (Internet Protocol) and PPP (Point to Point Protocol) are the two lowest-level protocols in normal dialup Internet access. (Many first-generation WAP devices work by making a normal dialup call and sending the WAP data across a modem link. IP and PPP are used in such a case.)

• SMS (Short Message Service) is a feature provided by most digital mobile phone systems that allows short messages to be sent and received across the wireless link. (Users see this as the ability to send short text messages - a bit like email.) It can also transmit binary data and be used for WAP.

• GPRS (General Packet Radio System) is a next-generation mobile phone system that will bring higher bandwidth and permanent network connections to cell phones and other wireless devices. Instead of having to dial into the server, a cell phone will effectively have a permanent Internet connection. GPRS is based on IP.
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